Electric vehicles are the vehicles propelled by electricity as opposed to the conventional vehicles which operate on fuels. Electric vehicles may use a combination of different energy sources such as for example, fuel cells (FCs), batteries, and super capacitors (or ultracapacitors) to power an electric drive system as shown in FIG. 1.
Electric vehicles (EVs) operate with higher energy conversion efficiency, produce a lower level of exhaust emissions, and lower levels of acoustic noise and vibration, than conventional vehicles.
The electricity needed tor Electric Vehicles operation can be produced either outside the vehicle and stored in a battery, or produced on-board with the help of fuel cells (FC's).
In EVs, the main energy source is assisted by one or more energy storage devices. Two often used energy storage devices are batteries and supercapacitors which can be connected to the fuel cell stack in many ways. The voltage characteristics of the two devices have to match perfectly, and only a fraction of the range of operation of the devices can be utilized, e.g., in a fuel cell/battery configuration, the fuel cell must provide almost the same power all the time due to the fixed voltage of the battery, and in a battery/super-capacitor configuration, only a fraction of the energy exchange capability of the supercapacitor (also referred to herein as ultracapacitor) can be used.
DC/DC converters can be used to interface the elements in the electric power train by both boosting or chopping the voltage levels as required by the load in different regimes of the EV operation. By introducing the DC/DC converters, one can choose the voltage variations of the devices, and the power of each device can be controlled.
Different configurations of EV power supplies dictate that at least one DC/DC converter is necessary to interface the FC, the battery, or the supercapacitor's module and the DC-link. The DC/DC converter structure has to be reliable and lightweight, small in volume, operate with high efficiency and low EMI (electro-magnetic interference) and low current/voltage ripple.
A DC/DC converter is a category of power converters having an electric circuit which converts a source of direct current (DC) from one voltage level to another, by storing the input energy temporarily and subsequently releasing the energy to the output at a different voltage.
The storage of the input energy may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors). DC/DC converters can be designed to transfer power in only one direction, from the input to the output. However, almost all DC/DC converter topologies can be made bi-directional which can move power in either direction, which is useful in applications requiring regenerative braking.
The amount of power flow between the input and the output in the DC/DC converter can be controlled by adjusting the duty cycle which is identified as a ratio of on/off time of a switch in the DC/DC converter. Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power.
A wide variety of DC/DC converter topologies have been developed. Some design considerations are essential in automotive applications for the DC/DC converters. These considerations include light weight, high efficiency, small volume, low intermagnetic interference, low current ripple drawn from the fuel cells or the battery, the step up function of the converter, and achieving control of the DC/DC converter power flow in the wide voltage variations on the converter input.
The most common DC/DC converters can be grouped in several categories:
Non-isolated converters which are generally used where the voltage needs to be stepped up or down by relatively small ratio (less than 4:1). There are five main types of converters in the non-isolated group, usually called the buck, boost, buck-boost, CUK, and charge-pump converters. The buck converters are used for voltage step down, while the boost converters are used for voltage step up. The buck-boost and CUK converters can be used for either step down or step up. The charge-pump converters are used for either voltage step up or voltage inversion, but only in relatively low power applications.
Another category of DC/DC converters includes isolated converters in which a high frequency transformer is used. These converters are useful in the applications where the output needs to be completely isolated from the input. All of these converters can be used as bi-directional converters with a high ratio of stepping down or stepping up the voltage. Transformer-based converters may provide isolation, between the input and the output.
Each converter topology has its advantages and its drawbacks. For example, the DC/DC boost converters may not meet the criteria of electrical isolation. Moreover, the large variance in magnitude between the input and the output may impose serious stresses on the switches in the converter. This topology also may suffer from high current and voltage ripples, and has a large volume and weight.
A basic interleaved multi-channel DC/DC converter topology permits reduction of the input and output current and voltage ripples, as well as reduction of the volume and weight of the inductors, and increase in the efficiency. This structure, however, cannot operate efficiently when a high voltage step-up ratio is required since the duty cycle is limited by the circuit impedance leading to a maximum step-up ratio of approximately 4. Hence, two series connected step-up converters would be required to achieve the specific voltage gain of the application specification.
A full-bridge DC/DC converter is the most frequently implemented circuit configuration for fuel cell power conditioning when electrical isolation is required. The full-bridge DC/DC converter is suitable for high-power transmission because switch voltage and current are not high. It has small input and output current voltage ripples. The full-bridge topology is a favorite for zero voltage switching (ZVS) pulse switch modulation (PSM) techniques. (Monzer Al Sakka, et al., “DC/DC converters for Electric Vehicles,” Electric Vehicles-Modeling and simulations, Dr. Seref Soylu (Ed), ISBN: 978-953-307-477-1, In Tech, available from: http://www.intechopen.com/books/electric-vehicles-modelled-and-simultations/adc-dc-converters-for-electric-vehicles).
Fast semiconductor devices make it possible to have high speed and high frequency switching in power electronic converters. High speed switching helps to reduce weight of the components and volume of equipment. However, it causes some undesirable effects, such as, for example, the radio frequency interference ignition. It is believed that high
            ⅆ      v              ⅆ      t        ⁢          ⁢  or  ⁢          ⁢            ⅆ      i              ⅆ      t      due to the modern power device switching is mainly responsible for the EMI emissions.
Another drawback of DC/DC converters is the operation with considerable energy losses including the losses produced by the semiconductor switches (IGBTs and diodes) and the passive components (capacitors and inductors).
It is desirable to provide a DC/DC converter for Hybrid Energy Storage Systems with minimized switching losses, lower inductor current ripple and low switch voltage ratings, as well as to reduce the output voltage ripples, and to enhance reliability and autonomity of the Hybrid Energy Storage Systems operation.